Cathode ray tube



Nov. 5, 1935 B. c. GARDNER CATHODE RAY TUBE Filed Jan. 28, 1935 INVENTOR,

5 I In m h l 4 a I Fania-Full 2 ATTORNEY Patented Nov. 1935 CATHODE RAY TUBE Bernard 0. Gardner, Philadelphia, Pa assignor to Television Laboratories, Ltd., San Francisco, Calii'., a corporation of California Application January 2811933, Serial No. 654,008

7 Claims.

My invention relates to luminescentmeans for use in cathode ray apparatus, such as oscilloscope or oscillographic tubes, and oscillights or television receiver tubes, this application being a continuation in part of the invention disclosed in the application of Philo T. Farnsworth, Serial No. 655,784, filed February 8, 1933.

Among the objects of my invention are:

' To provide a luminescent screen giving a pure white light, said screen being in the form of a thin, flexible metal foil.

To provide means for placing a metal foil luminescent screen under tension.

To provide means for maintaining a flexible metal foil under tension while being used as a target for a beam of cathode rays.

To provide a mounting for a cathode ray screen, said screen comprising a flat sheet of flexible metal foil.

To provide means for compensating for the expansion and contraction of a flexible metal foil when used as a target for a beam of cathode rays.

To provide a mounting for a. metal foil screen for cathode ray tubes which may be used to conduct current from an exterior source through said foil. I

To provide means for compensating for dimensional changes due to heat in a flexible metal foil used as a target for a beam of cathode rays.

Other objects of my invention will be apparent or will be specifically pointed out in the description forming a part of this specification, but I do not limit myself to the embodiment of my invention herein described, as various forms may be adopted within the scope of the claims.

Referring to the drawing:

Figure 1 is a longitudinal "sectional view through an oscillight or cathode ray tube embodying this invention.

Figure 2 is a cross sectional view taken as indicated by the line 2-2 in Figure 1.

It has been shown in application, Serial No. 655,784, filed February 8, 1933, referred to above, that a thin sheet of refractory metal foil may be bombarded by a moving beam of cathode rays to raise successive elementary areas of the foil to incandescence. thus giving rise to a visible trace on the screen, this trace having extreme brilliancy and highly desirable color values, particularly for theformation of effective oscillographic and television images.

It has also been pointed out in the application above referred to that it is desirable to polarize the foil by supplying a steady heat to the screen during the traversal by the cathode ray beam, this heat being preferably obtained by resistively heating the metal foil by passing an electrical current through it.

This invention is directed toward the mounting of such metal foil screens, the mounting being 5 disclosed in the above mentioned application, but not claimed therein.

In the operation of a cathode ray tube equipped with a metal foil target, when the target is polarized by steady heat in addition to the heat 10 produced by the cathode ray beam, there will be expansions and contractions of the foil due to the applied heat, the amount of change in physical dimensions being of course dependent upon the amount of heat and the particular metal used. 15 There will be a change in dimension due to the steady polarization, and a further change, constantly varying in amount, due to the usual variations in the power contained in the moving beam. Such changes in dimension, will, if not compen- 20 sated for, buckle the foil and distort the resultant image. Furthermore, as cathode ray beams are usually focused on the target in a fine spot, any change in distance between the source of the beam and the target will tend to enlarge the spot 25 and thus reduce element definition in the image.

My invention comprises broadly the placing of a metal foil cathode ray screen under tension, preferably by applying a stress across a major dimension of the screen when cold, so that the 30 foil surface will remain substantially flat when heated by the polarizing source, and also when still further heated by the cathode ray beam. The screen therefore cannot buckle, and always remains substantially in the plane of focus of the 35 cathode ray beam. The means for placing the foil under tension are also preferably designed so that a current from an exterior source may be brought to the screen for resistive heating. It

is to be understood, however, that the need for 40 tension exists entirely apart from the polarizing heat, because if no polarizing heat be separately applied, an amount of power equivalent to a separate polarizing power must be included in the cathode ray beam to bring the screen up to the 45 proper operating temperature, and the total amount of dimensional change in the screen will be the same in both cases.

The use of a separate polarizing heat however with the screen under tension is desirable, not 50 only from the point of view of saving power in the cathode ray beam, but also in partially expanding the screen before the cathode ray beam starts to operate. For example, if the screen is to be used to produce oscillographic traces without 55 modulation or with rectified beam impulses, without polarization, the screen will be raised from cold to incandescence with full physical distortion of the foil, due to the action of the beam alone, whereas when polarization is useda much smaller distortion will take place during the formation of the image by the beam. Although the tensioning means will fully compensate both distortions if prolonged, it is obvious that more faithful reproduction will be obtained when the foil is first The screen itself should preferably be of a refractory metal capable of being raised to incandescence by the impact of the beam, and it has been found that tantalum foil formed as described in the application of Gardner and Varian, 5 Serial Number 637,772, filed October 14, 1932, is entirely satisfactory. A suitable thickness of foil hasbeenfoundtobefromwto 100x iii-inches. but the inventionhere described is applicable to any foil which is flexible and capable of being 10 partially expanded, because of the time element, heated toincandescence by the cathode ray beam.

involved. It is often possible when the screen is heated from cold to inoandesoence by the bmm alone that aplanar distortion may occur and end before the tension device can operate. Partially expanding the screen greatly reduces such fleeting aplanar distortion bygreatly reducing the amount of expansion possible due to. the heat of the beam alone.

The form of the invention illustrated in the drawing comprises the usual evacuawd envelope I having at one end a reentrant stem I which carries anelectrongun. The type of gun used is preferably that shown in detail in the copending application of Gardner and Brolly, Serial Number 614,500, filed May 31,1932, this form of cathode ray projector being capable of producing a cathoderaybeamofhighintensity. 'Ihecathode (not shown) is heated by current supplied by a source, through leads 4 and i, emitting electrons which are attracted by aconical projection ton acircularanodelandpassthrough-abeamcanal intheanodetoformtherequiredbeamofcathode rays. 'lheanodeisweldedtoasupportl,thisin turnbeingsupportedfromaclampring II. A

-leadwirellconnectstotheanodesupportandis film of metal deposited from the cup II. This film being energized at anode potential and covering the barrel of the tube, protects-the beam from the influence of extraneous potentials.

The end of the envelope opponte the gun is slightly expanded to form a screen chamber II into which is sealed a heat screen assembly supported by a screen stem 2!. This screen assembly preferably comprises a rectangular wire frame 24 mounted on frame leads 2|. A cross bar 20 provides additional strength. Insulating sliders 21 of lava or similar material are connected by a mounting bar as and are movable along the frame work. A target comprising a heat screen ll of any desired shape, preferably rectangular. is welded to the top of the frame, and to the mounting bar. Springs ll welded at one end to the frame press against the sliders I1 and maintain the screen II under tension. Flexible leads 3! connectthemountingbartoscreenleadsflsealed through the stem 2!. A circuit is thus established so that a source of current exterior to the tube maybeusedtoheatthescreen.-andexpanditto a predetermined amount, the springs forcing the mounting bar to take up the expansion and greatly reduce the aplanar distortion.

Whensuchascreenashasbeendescribedis heated it will change its physical dimensions, usually, in the case of most refractory metals, it willexpand. Iftheexpansionisnotcompensated 15 for, the screen will buckle, either toward or away from the electron gun, and the point of impact of the beam. focused to a predetermined siae, usuallyveryfine.willbecloserorfartheraway fro'mthegun. Suchavariationindistanoere-zo duces detail. often changesthesiseofthespotand in general is undesirable. By placing the foil under tension this aplanar distortion may be reduced to such an extent as to be a negligible factor in the production of an eifective image. as

Whenapolarisingheatisusedonanuntensioned foil, the application of the polarizing heat alone will distort the screen from its flat shape, andthecathoderaybeamwillcausestillgreater distortion. Byplacingthefoilundertensionthefl distortioncausedbythepolarisingheatislargely takenupbeforethecathoderaybeginstoopesate, andafterthebeamdoesstart,thespringsecmpens'ate for the additional expansionandoontractionduetothebeamheat. s

Inthe tubeshown,onlytheexpansioninone ItwilLhowevenbeobviousthattm- Iclaim:

apparatus comprising an envelopaanelectrongumatargetforsaidgun formedfromthinmetallicfoilofsubstantially rectangular dimensions, means for WM saidfoilinadeflniteplaneoomprisingafrsmework to which one edge of said foil is attached.

a movable member positioned by said framework towhich the oppositeedgeofsaid foilisattached, and mans for applying stress between said framework and said movable member to compensateforexpansionandcontractionofsaid foil 2: A cathode ray apparatus comprising an ensaid framework, and means for conducting current from outside said envelope to said foil through said framework and said movable memher.

3. The method of reducing aplanar distortion of a substantially planar sheet of thermally expansive material when used as a target of a beam of cathode rays which comprises applying a tensioning stress in the plane of the sheet, expanding said material a predetermined amount, and exposing said partially expanded material to said beam.

4. A cathode ray tube comprising an envelope, a cathode and anode cooperating to form a beam of catnode rays therein, a luminescent screen for said beam consisting of a thin sheet of flexible metal foil, and means for applying a stretching stress to opposite edges of said foil to maintain said foil in a plane during bombardment by said beam.

5. A cathode ray tube comprising'an envelope, a cathode and anode cooperating to form a beam of cathode rays therein, a luminescent screen for said beam consisting of a thin sheet of flexible metal foil, means for stiffening opposite edges of said foil, and means for applying a stress beof .cathode rays therein, a luminescent screen 5 for said beam conslstingof -a thin sheet of flexible metaltoil, means for stifiening opposite edges of said foil, means for applying a stressibetween said stifiening means to place said foil under tension in a plane, means for insulating said stiffening means from each other, and means for passing a current through said stiffening means and said foil.

7. A cathode ray tube comprising an envelope, a cathode and anode cooperating to form a beam of cathode rays therein, a luminescent screen for said beam consisting of a thin sheet of flexible metal foil, means for stiffening opposite edges of said foil, means for applying a stress between said stiffening means to place said foil under tension in a plane, means for insulating said stifiening means from each other, and an electrical connection from without said envelope to each of said stiffening means.

BERNARD C. GARDNER. 

